Liquid level detectors are used in various fields with different purposes and different modes of operation. Liquid level detectors are particularly used in so-called pipetting channels used in laboratory liquid handling equipment, wherein even very small amounts of liquids have to be precisely detected/measured.
One particular type of liquid level detection is known as capacitive liquid level detection, wherein the presence or the level of a liquid is detected by monitoring for sudden changes of the capacitance seen between the tip and the grounded work surface, these sudden changes in capacitance occurring due to physical contact between tip and the liquid being established or interrupted.
Various circuits have been developed in order to be able to provide an output signal indicating whether a fluid is present or its level based on this change of input capacitance. One particular circuit, well established in the field, works by the principle of charging the capacitor, which includes the wanted liquid capacitance as well as the unwanted cable and filtering circuit capacitance followed by a discharge. The time needed to charge the total present capacitance at the input is converted in a pulse width modulation duty cycle proportional to the input capacitance. The following band pass filter and amplifier removes the DC component and amplifies only the voltage change resulting in a voltage pulse proportional to the capacitance change at the sensor input. If said pulse would exceed a predetermined voltage, a threshold value, an output signal would be generated. A clock signal is used to control the charge/discharge cycles. The frequency of the clock signal is directly proportional to the sensitivity of the liquid level detection. Thus altering the frequency has to be compensated either with an alteration of the threshold value or by changing the amplification gain of a signal amplifier to compensate for the amplitude change of the liquid detection input signal.
In classical applications, the clock frequency was preset and fixed in production stage for these circuits and a corresponding reference voltage and/or amplification gain was calculated and fixed so that the required sensitivity is ensured. However, certain applications require that the clock frequency is changed. As a consequence, the reference voltage and/or amplification gain has to be also changed accordingly, otherwise the sensitivity will be affected and false outputs would be generated.
One of the reasons the clock frequency has to be changed is that these capacitive Liquid Level Detectors and their control circuitry are very sensitive to interferences. Such interferences can be caused by other electronic apparatus in the proximity which operates at a similar or identical frequency as the clock signal. As with all interferences, this problem becomes worse as the distance between the two devices operating at nearby frequencies gets smaller. This is particularly the case in so-called multi-axes or multi-channel configurations of laboratory equipment, where multiple liquid level detectors are mounted on said multiple axes.
Existing designs for capacitive liquid level detection, where multiple capacitive liquid level detectors are in close proximity come along with the need to either synchronizing all capacitive liquid level detectors within a system or to set the clocking frequencies apart from each other to prevent from interference which could lead to false detection.
Synchronization of all capacitive liquid level detectors represents only an acceptable solution if all axes are fix positioned in relation to each other. All other cases where those axes are independently positioned introduce difficulties due to wiring constraints and possible electromagnetic interference issues. For a synchronization of all capacitive liquid level detectors, an additional cross connection between them would be needed or a central clock generator to be implemented on an interconnection board or the like.
The second approach for preventing interferences, i.e. setting the frequencies apart from each other has the consequence that sensitivity will change accordingly. This could be compensated using digital signal processing or, in case of level comparators by adjusting the reference voltage and/or amplifier gain. Adjusting the reference voltage and/or amplification gain can be accomplished with different assemblies for each frequency setting by some sort of switch or jumper settings or by a programmable voltage reference. However, adjustment of the amplification gain has the disadvantage that it is highly dependent on the accuracy and linearity of the amplifier in all amplifying domains. All these solutions add cost and/or risk of wrong sensitivity settings leading to potential misleading results without error visibility.
Objective of the Invention
Thus, the objective of the present invention is to provide a method for liquid level detection which is able to provide a reliable output signal indicting the presence of a liquid, said method being insensitive to changes in the clock frequency, said method requiring neither additional digital signal processing nor individual setting of the reference voltage and/or amplification gaining respect of each frequency change but at the same time allowing frequency independent sensitivity adjustments.
A further objective of the present invention is to provide a method for multi-channel liquid detection wherein interferences between said multiple channels are prevented, said method requiring neither additional digital signal processing nor individual setting of the reference voltage and/or amplification gaining respect of each frequency change.
An even further objective of the present invention is to provide a liquid level detection unit for capacitive liquid level detection which is capable of providing a reliable output signal indicting the presence of a liquid, and which is insensitive to changes in the clock frequency but at the same time maintains a preset sensitivity, without the need for neither additional digital signal processing nor individual setting of the reference voltage and/or amplification gaining respect of each frequency change.
An even further objective of the present invention is to provide a multi-channel liquid level detection system, wherein interferences between multiple liquid level detection units are prevented and wherein the sensitivity of each channel can be individually set, said sensitivity not being affected by frequency changes.